The spinal column is formed from a number of bony vertebral bodies separated by intervertebral discs which primarily serve as a mechanical cushion between the vertebral bones, permitting controlled motions (flexion, extension, lateral bending and axial rotation) within vertebral segments. The normal, natural intervertebral disc is comprised of three components: the nucleus pulposus (“nucleus”), the annulus fibrosis (“annulus”), and two opposing vertebral end plates.
The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body.
The nucleus is constituted of a gel-like substance having a high (about 80-85%) water content, with the remainder made up mostly of proteoglycan, type II collagen fibers and elastin fibers. The proteoglycan functions to trap and hold the water, which is what gives the nucleus its strength and resiliency.
The annulus is an outer fibrous ring of collagen fibers that surrounds the nucleus and binds together adjacent vertebrae. The fibers of the annulus consist of 15 to 25 overlapping collagen sheets, called lamellae, which are held together by proteoglycans. The collagen fibers that form each lamellae run parallel at about a 65° angle to the sagittal plane; however, the fibers of adjacent lamellae run in opposite directions from each other. As such, half of the angulated fibers will tighten when the vertebrae rotate in either direction. This configuration greatly increases the shear strength of the annulus helping it to resist torsional motion. The annulus has a height of about 10 to 15 mm and a thickness of about 15 to 20 millimeters, occupying about ⅔ of the intervertebral space.
With aging and continued stressing, the nucleus becomes dehydrated and/or one or more rents or fissures may form in the annulus of the disc. Such fissures may progress to larger tears which allow the gelatinous material of the nucleus to migrate into the outer aspects of the annulus which may cause a localized bulge or herniation. In the event of annulus rupture, the nuclear material may escape, causing chemical irritation and inflammation of the nerve roots.
Posterior protrusions of intervertebral discs are particularly problematic since the nerve roots are posteriorly positioned relative to the intervertebral discs. Impingement or irritation of the nerve roots not only results in pain in the region of the back adjacent the disc, but may also cause radicular pain such as sciatica. Nerve compression and inflammation may also lead to numbness, weakness, and in late stages, paralysis and muscle atrophy, and/or bladder and bowel incontinence.
The most common treatment for a disc protrusion or herniation is discectomy. This procedure involves removal of the protruding portion of the nucleus and, most often, the annular defect does not get repaired. Typically, removal of the nucleus material is accomplished through the herniation site or a weakened portion of the annulus.
Discectomy procedures have an inherent risk since the portion of the disc to be removed is immediately adjacent the nerve root and any damage to the nerve root is clearly undesirable. Further, the long-term success of discectomy procedures is not always certain due to the loss of nucleus polposus which can lead to a loss in disc height. Loss of disc height increases loading on the facet joints which can result in deterioration of the joint and lead to osteoarthritis and ultimately to foraminal stenosis, pinching the nerve root. Loss of disc height also increases the load on the annulus as well. As the annulus fibrosis has been shown to have limited healing capacity subsequent to discectomy. A compromised annulus may lead to accelerated disc degeneration which may require spinal interbody fusion or total disc replacement.
If disc degeneration has not yet resulted in excessive herniation or rupture of the annulus, it may be desirable to perform a nucleus replacement procedure in which the degenerated nucleus is supplemented or augmented with a prosthesis while leaving the annulus intact. Advances have been made in materials for prosthetic nuclear implants which are relatively small and flexible (e.g., hydrogels), and are able to provide added height to the disc while simulating the natural disc physiology and motion. However, in order to implant a prosthesis within the nucleus cavity, an appropriately sized passageway through the annulus (i.e., an annulotomy) must be created. As with naturally-occurring defects in the annulus, the resulting surgical annulus defect may lead to post-implant complications. Currently accepted suturing techniques are of minimal value in light of the forces normally exerted on the annulus, including an inability to adequately resist explant of the nuclear implant.
Various annular defect repair techniques have been developed to occlude an aperture, whether surgically or naturally formed, within the annulus, which attempt to address the shortcomings of suturing. Many of these techniques include the implantation of devices, such as patches, membranes, stents and the like, to form a barrier across the annulus aperture in order to seal or occlude the aperture and/or to prevent explant of native or prosthetic nuclear material. While an improvement over conventional suturing, these annulus implants and repair techniques are limited in their ability to provide the extent of circumferential and radial competency to the annulus for long-term success. Additionally, where the disc repair procedure also involves the implantation of both annulus and nucleus augmentation devices, the implants and the steps necessary to implant both may counter-indicate each other.
Accordingly, it would be highly advantageous to be able to repair a degenerating or ruptured disc in a manner which obviates the inherent risks of discectomy procedures, and which augments the nucleus and/or annulus in a way that reduces the risk of re-herniation of the disc subsequent to repair. Additionally, it would be highly beneficial to provide a technique which allows disc repair in a minimally invasive requiring minimal steps and instrumentation to perform both annuloplasty and/or nucleus replacement procedures concurrently in a synergistic manner.